Yeah, we iterated a bunch on this question, and I still don't think we have it quite right.
On the REPL side, we've had a few prototyped out [1] out over the years. We'd love something someday. What do you use for your scientific computing? We are also have some projects that are trying to ease using Rust with other languages ([2], [3]), so knowing what we should priorities would help.
[1]: https://github.com/murarth/rusti